After a hydrocarbon well has been drilled and made safe, various operations are generally carried out:                well testing operations (serving to characterize the various components of the effluent flowing out of well, and to estimate the production capacities of the well), and subsequently        well producing operation as long as the oil produced is satisfactory in term of quality, flowing rate, etc . . . .        
During these operations, the composition of the effluent varies. The effluent that is initially collected is essentially made up of water. Subsequently, the percentage of aqueous residue decreases gradually, and the composition of the effluent becomes enriched with oil and with gas. Thus, the effluent is a multiphase fluid mixture. In addition, the multiphase fluid mixture may have complex flow regimes, e.g. mist, bubble, slug, churn flows, etc . . . in the well-bore or in the flowing lines.
During the testing and producing operation, an important concern is to identify and/or analyze as accurately as possible the various phases that constitute the multiphase fluid mixture. A preliminary step is to obtain a representative sample of the multiphase fluid mixture flowing out of the hydrocarbon well. Prior art systems disclosed in patents U.S. Pat. Nos. 6,182,505 and 6,212,948 relate to such sampling apparatus.
For sampling a phase from a multiphase fluid mixture, it is known to provide the pipe in which the multiphase fluid mixture flows with gravity traps (upper and lower traps) coupled to appropriate drain for collecting a particular phase. The representation of the phases collected by the gravity traps is questionable because the phases in the gravity traps are not refreshed continuously. In addition, the lower trap collects all heavy phases such as liquids (oil, water) and solids. These solids often plug the drain preventing further sampling, and if the heaviest liquid flow (water) is important, it is very difficult to drain the lighter one (oil) as the accumulation rate could be faster than the one draining.
It is also known to take a single shot sample (sampling cylinder) from the pipe in which the multiphase fluid mixture flows, to recondition the sample and to separate the phase later in a laboratory. The sample reconditioning in a laboratory consists in applying the pipe temperature and pressure for reaching the original thermodynamic equilibrium and therefore the original fluid phase compositions. The stabilized phases are then separated and transferred in several cells for physico-chemical measurements. This method does not enable to control the sampled volume of each phase, which could lead to an insufficient quantity of a given phase for the post-sampling measurements.